Study Suggests Ongoing Rehab Is Necessary for Significant Inflow and Infiltration Reduction

12-year study shows that effectiveness of rehabilitation deteriorates if it isn’t continually maintained

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After Nashville (Tennessee) Metro Water Services completed a sewer rehabilitation project in 1996, a long-term study showed that rehabilitation effectiveness decreased 7% over 12 years.

Early results showed 620 million gallons of infiltration, or approximately 35% of inflow and infiltration, were removed annually. At the end of the study, that number was down to about 28%, or 488 million gallons.

“The results could suggest that the level of effectiveness has deteriorated slightly in the past 10 years,” the study says. “However, it may also reflect the effect of continued deterioration of parts of the system that were not rehabilitated.”

It is important to note that even the later decreased removal numbers maintained the project’s original mitigation goals.

The project was also solely cured-in-place pipe relining and didn’t encompass holistic mitigation, meaning the decrease could also be contributed to the limitations of a single-method approach. That being said, the study did not find any specific or concrete failings of the CIPP rehab.

“At the time of this project, inverting the saturated CIP material into the host pipe resulted in direct exposure of the resin-saturated felt to the inner surface of the deteriorated pipe,” the study says. “Due to the tight fit, and direct resin contact between the host pipe and liner, the follow-up inspections revealed that there were generally no annular spaces for I&I to ‘track’ down to the next available manhole.”

Taking a regional approach, another factor is that the CIPP rehabilitation only targeted a single area with significant I&I, without addressing larger systemwide concerns.

Entitled “Nashville Project Shows Long-Term Effectiveness of Sewer Rehabilitation for Infiltration Reduction,” the paper’s authors — George E. Kurz, P.E., DEE; Gregory A. Ballard, P.E.; and Leanne B. Scott, P.E. — sought to answer the question, “What is the life span for these (rehabilitation) products, and how long will they remain effective for reducing groundwater infiltration as originally designed?”

They found that although it is likely the CIPP liner had a reduced effectiveness in the years following its installation, it is unlikely that was the only or even the most-significant factor explaining the drop in removal rates over time.

As this study began in 1996, it also did not take into account I&I additions from system expansions over the decade that it was analyzing.

Data was gathered via three permanent ADS Environmental Services flowmeters installed in 1990, as well as 16 temporary flowmeters installed in various subbasins, gathering data over four separate instances for the scope of the study, which ended in 2007.

Over 50 miles of 8- to 30-inch sewer lines, serving light commercial districts and neighborhoods of expensive older homes, small estates and upscale cluster homes, were part of the rehabilitation effort. It also addressed 315 service laterals. Four separate contracts within the ‘Sugartree’ project relined 42,000 linear feet, about 15.5% of the overall system.

“The design goal established for the Nashville Overflow Abatement Program was to reduce I&I to a level where overflows in residential or sensitive areas would not occur more frequently than once in five years,” the authors say. “Statistics show that the largest 24-hour rainfall likely to occur at least once in five years is 4.5 inches. Therefore, to satisfy the design goal, the sewer system must convey all flow resulting from 4.5 inches of rain in 24 hours.”

Though the study did not ultimately conclude whether there were issues with the life span or long-term mitigation effects of the particular rehabilitation method under analysis, it did create a baseline to build on in future studies.

“Unsatisfactory I&I reduction following rehabilitation in some communities suggests that the process of groundwater migration from sealed areas to the next available defect is not well understood,” the paper says. “This study cannot be considered a conclusive answer … regarding life-cycle effectiveness of sewer rehabilitation for I&I removal. However, it may illustrate a general approach for conducting additional studies. In particular, the procedures for I&I analysis are critical and should be carefully evaluated for reproducibility and quality control.”



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